Make the edge of a model smoother
You could draw a curve with grease pencil(d), making sure it would hold the part that needs to be cut away inside if it was closed, convert it to a curve select it and then the object so the object is active and enter edit mode and just use knife project. You don't need to use grease pencil and could create the curve some other way as well. 

 

Grease pencil smoothing functionality might also be useful: